Description
Renders World lists the below DPC insulation board 90mm as a single-board line for plinth, foundation and basement perimeter work, one of five depths in the XPS plinth and below-DPC board range.
Where the 90 mm Board Sits in a Plinth Build-Up
The XPS foundation board 90mm declares a thermal resistance of 2.50 m²K/W at λD 0.036 W/mK, enough to insulate a plinth and basement perimeter as one continuous plane from footing level to the damp-proof course.
That 2.50 figure is the manufacturer's declared resistance for the 90 mm thickness rather than thickness divided by conductivity, which is why it is the value an assessor can take straight into a build-up calculation. Where the elevation above the damp-proof course is insulated to the same depth, the two planes line through and the reinforced layer wraps the junction without a step to compensate for.
The data sheet sets four intended positions for the board, and all four are ground-contact or ground-adjacent rather than facade work:
- External foundation and basement walls below ground level, in normally loaded positions where backfill gives the wrap permanent restraint.
- Foundation walls and plinths above ground level, carrying the render build-up down into the splash zone.
- Terraces, beneath the waterproofing and the finish layers.
- Ground-bearing floors in normally loaded positions, laid over a level slab ahead of screed.
Why Specifiers Order 90 mm Rather Than the Next Depth Down
Four declared figures decide this depth, and each one changes how the board is ordered or fitted.
- 2.50 m²K/W declared resistance — 0.30 m²K/W more than the 80 mm board for 10 mm of extra build-up depth, which keeps paving levels and thresholds workable.
- λD 0.036 W/mK — a low conductivity for a moisture-tolerant board, so the plinth reaches its target without stepping out beyond the facade line.
- 100 kPa compressive strength at 10% deformation, so the board takes backfill pressure and paving loads at the plinth as part of the build-up.
- No more than 3% water absorption under long-term total immersion, which is what qualifies it for the wet and periodically saturated positions the sheet names.
What the Data Sheet Declares at 90 mm
Every figure declared for the XPS foundation board 90mm was read from the current manufacturer data sheet during this session, while the board format was read from the live listing rather than from the sheet.
| Property | Declared value | What it means on site |
|---|---|---|
| Thickness | 90 mm | Plinth depth that lines through with a 90 mm facade board |
| Board format | 1000 × 500 mm | Two boards per square metre; sold as a single board |
| Thermal conductivity | λD 0.036 W/mK | Low conductivity for a moisture-tolerant board |
| Thermal resistance | 2.50 m²K/W | Declared for 90 mm; the value a U-value calculation takes |
| Compressive strength | 100 kPa at 10% deformation | Takes backfill and paving loads at the plinth |
| Water absorption | 3% maximum, long-term total immersion | Suits wet and periodically saturated positions |
| Reaction to fire | Class E | The board's own class; a rendered build-up is classified separately |
One point is worth settling before the order goes in. The sheet describes the product in both a smooth-edged and a rebated form and does not say which reaches this shelf, so ask at order and detail the joints as plain butt joints until the rebate is confirmed.
Fitting the 90 mm Board Below and Above Ground Level
Installation follows the sheet's own sequence, and the separating layer between board and soil is the step that repays the most attention.
- Set the datum first. Strike a level line at the damp-proof course and work down, so the plinth courses reach footing level without a tapered cut.
- Bond with a compatible adhesive. The sheet names cement-based adhesives, polyurethane foams and bituminous compounds based on water dispersion; the cement-based route sits on the EPS adhesives basecoats shelf.
- Keep organic solvents away from the board. The same sheet asks for storage clear of solvents and their vapours, which is the reason a water-dispersion bituminous product is specified rather than a solvent-based one.
- Separate the board from the ground. Below ground level the sheet calls for a separating layer — a reinforced layer with damp-proofing or waterproofing over it, or a dimpled membrane — before anything is backfilled. Vapour transmission and freeze-thaw resistance are left undeclared on this product, so that layer is what manages ground water, and it is a detail worth drawing rather than improvising.
- Backfill in even lifts. Compacting in stages keeps pressure balanced along the wrap and gives the 100 kPa board the restraint it is specified for.
- Fix mechanically above grade. Above ground the plinth carries wind load, and both plug length and pattern come from the system holder's fixing table for that substrate; the plugs, discs and base tracks sit in the insulation fixing accessories range.
- Continue above the damp-proof course in facade board. Graphite EPS insulation boards take the elevation, where render compatibility and vapour behaviour matter more than immersion performance.
Store the boards flat, protected from mechanical damage and away from ignition sources, as the sheet instructs. Renders World supplies the same 1000 × 500 mm format across all five depths, so a mixed-thickness plinth still sets out on one grid.
The 90 mm Board Against the Other Four Depths
| Depth | Declared resistance | Where it fits |
|---|---|---|
| 30 mm board | 0.80 m²K/W | Tight perimeter runs and shallow returns |
| 50 mm board | 1.35 m²K/W | Floor build-ups and shallow foundation wraps |
| 80 mm board | 2.20 m²K/W | Plinth matched to an 80 mm facade board |
| 90 mm board | 2.50 m²K/W | Below-DPC work matched to a 90 mm facade |
| 100 mm board | 2.75 m²K/W | Deepest wrap and the plinth-to-wall transition |
The steps between those figures are uneven because each one is a declared value rather than a calculation, which is worth knowing when a specification is written to a resistance target. For the wider material question, the guide to XPS and EPS boards compared for UK walls works through where each belongs, and the plinth thermal bridge and mould prevention guide covers the overlap detail at the damp-proof course.
Ordering 90 mm — Price per Board, Quantities and Companions
The XPS foundation board 90mm is sold as a single board at £9.50 per board, as displayed on the product page in August 2026, and was listed as available at the time of that fetch.
- Boards per square metre: each 1000 × 500 mm board covers 0.5 m², so 1 ÷ 0.5 = 2 boards per m², and a 20 m² plinth run is 20 × 2 = 40 boards before an allowance for cuts at corners and returns.
- Material cost: £9.50 ÷ 0.5 m² = £19.00 per m² for the board alone, before adhesive, fixings and the render build-up.
- Plinth finish: a mosaic render finish for plinths is the usual decorative close for the zone above ground.
Renders World lists the adhesive, the fixings and the plinth finish as separate lines rather than a bundled rate, so the take-off stays visible on the quotation.
FAQ — Depth Choice, Adhesives and Below-Ground Detailing
How many 90 mm boards are needed per square metre?
Two. Each 1000 × 500 mm board covers 0.5 m², so 1 ÷ 0.5 = 2 boards, and a plinth 24 m in length at 500 mm high is 12 m², or 24 boards before any allowance for cuts.
Which adhesive suits this board below ground?
The data sheet names three routes — cement-based adhesives, polyurethane foams, and bituminous compounds based on water dispersion. Below ground the adhesive is only part of the detail: the separating layer and the staged backfill are what hold the wrap in position long term.
Can this board be used on the facade above the damp-proof course?
Its declared strengths are immersion and load performance, which belong to the plinth, foundation and basement perimeter. Above the damp-proof course the facade is better served by a graphite board designed for adhesive-and-basecoat bonding and a mesh-reinforced render, with the 90 mm depth carried through so the two planes stay flush.
How does the 90 mm compare with the 100 mm board?
Declared resistance rises from 2.50 to 2.75 m²K/W, a gain of 0.25 m²K/W for 10 mm of extra depth. Where paving levels or a matching facade thickness fix the plinth depth, 90 mm is the working choice; where the wrap is free to grow, the 100 mm foundation board takes the deeper target.
What does reaction to fire class E mean for a rendered plinth?
Class E is declared for the board on its own and stays a board property. The rendered build-up over it is a different assembly with its own test evidence, so a plinth specification takes its performance from the system holder rather than from this figure.
Technical Documentation
- HYDROSTYR 100 technical data sheet (GENDERKA, PDF) — declared conductivity, resistance by thickness, compressive strength, water absorption, reaction to fire, and the installation and storage instructions quoted above.
- The declaration of performance is confirmed at order, as the supplying plant determines which declaration applies.